1. Technical Field
The present invention relates to a high-brightness color liquid crystal display (LCD) panel with improved image contrast employing non-absorptive spectral filtering, light recycling among neighboring subpixels and ambient glare reduction, and also to methods and apparatus for manufacturing the same.
2. Brief Description of the Prior Art
Without question, there is a great need for flat display panels capable of displaying video imagery in both direct and projection modes of viewing. Examples of equipment requiring such display structures for direct viewing include notebook computers, laptop computers, a and palmtop computers, and equipment requiring such display structures for projection viewing include LCD projection panels and LCD image projectors.
In general, prior art color LCD display panels have essentially the same basic construction in that each comprises the following basic components, namely: a backlighting structure for producing a plane of uniform intensity backlighting; an electrically-addressable array of spatial intensity modulating elements for modulating the spatial intensity of the plane of backlight transmitted therethrough; and an array of color filtering elements in registration with the array of spatial intensity modulating elements, for spectral filtering the intensity modulated light rays transmitted therethrough, to form a color image for either direct or projection viewing. Examples of such prior art LCD panel systems are described in xe2x80x9cA Systems Approach to Color Filters for Flat-Panel Displaysxe2x80x9d by J. Hunninghake, et al, published in SID 94 DIGEST (pages 407-410), incorporated herein by reference.
In color LCD panel design, the goal is to maximize the percentage of light transmitted from the backlighting structure through the color filtering array. However, using prior art design techniques, it has been impossible to achieve this design goal due to significant losses in light transmission caused by the following factors, namely: absorption of light energy due to absorption-type polarizers used in the LCD panels;xe2x80x94absorption of light reflected off thin-film transistors (TFTs) and wiring of the pixelated spatial intensity modulation arrays used in the LCD panels; absorption of light by pigments used in the spectral filters of the LCD panels; absorption of light energy by the black-matrix used to spatially separate the subpixel filters in the LCD panel in order to enhance image contrast; and Fresnel losses due to the mismatching of refractive indices between layers within the LCD panels. As a result of such design factors, the light transmission efficiency of prior art color LCD panels is typically no more than 5%. Consequently, up to 95% of the light produced by the backlighting structure is converted into heat across the LCD panel. Thus, it is impossible to produce high brightness images from prior art color LCD panels used in either direct or projection display systems without using ultra-high intensity backlighting sources which require high power supplies, and produce great amounts of heat necessitating cooling measures and the like.
The light transmission efficiency of prior art LCD panels has been severely degraded as a result of the following factors: absorption of light energy due to absorption-type polarizers used in the LCD panels; absorption of light reflected off thin-film transistors (TFTs) and wiring of the pixelated spatial intensity modulation arrays used in the LCD panels; absorption of light by pigments used in the spectral filters of the LCD panels; absorption of light energy by the black-matrix used to spatially separate the subpixel filters in the LCD panel in order to enhance image contrast; and Fresnel losses due to the mismatching of refractive indices between layers within the LCD panels. As a result of such light energy losses, it has been virtually impossible to improve the light transmission efficiency of prior art LCD panels beyond about 5%.
In response to the shortcomings and drawbacks of prior art color LCD panel designs, several alternative approaches have been proposed in order to improve the light transmission efficiency of the panel and thus the brightness of images produced therefrom.
For example, U.S. Pat. No. 5,822,029 entitled xe2x80x9cIllumination System and Display Devicexe2x80x9d discloses a LCD panel construction comprising a broad-band CLC reflective polarizer (32) disposed between the backlighting structure (10, 12, 30 and 34) and a reflective color filtering structure (18 in FIG. 1A) made from a pair of cholesteric liquid crystal (CLC) film layers, as shown in FIGS. 1A and 1B of the accompanying Drawings which are identical to FIGS. 5 and 6 in U.S. Pat. No. 5,822,029. As shown in FIG. 1A, the reflective color filter structure (18 in FIG. 1A) has a first layer with portions which reflect red, green and blue light while transmitting other colors ,and a second layer identical to the first layer but out of alignment therewith so that each region of the spectral filter transmits only one color of light from the light source of backlighting structure (illustrated in FIG. 1C), while all other colors are reflected back towards the backlighting structure. During operation, the spectral components which are not transmitted through its respective subpixel structure, are reflected back through the broad-band CLC reflective polarizer (32) and recycled within the backlighting structure, after polarization state conversion in order to improve the light transmission efficiency of the LCD panel and thus the output brightness thereof.
In order to recycle light striking the TFT areas and wiring regions associated with each subpixel region on the LCD panel, U.S. Pat. No. 5,822,029 discloses the use of a reflective-type black matrix about the transmission apertures of the subpixels, realized placing a reflective material and a quarter-wave film (36 and 38 in FIG. 1B) on a substrate attached to the layers of CLC reflective material.
In order to improve the viewing angle of the LCD panel, U.S. Pat. No. 5,822,029 also discloses in Col. 4, at lines 29-50 thereof, that a collimated light source m ay be used so that light emitted by the light source will fall within the angular acceptance bandwidth of the broadband CLC polarizer (32),
While U.S. Pat. No. 5,822,029 discloses an LCD panel construction having the above-described improvements, this prior art LCD panel system nevertheless suffers from a number of significant shortcomings and drawbacks. For example, the manufacturing of CLC layers having three color regions or sections is quite difficult because of the limited dynamic range in color tuning afforded by the CLC manufacturing techniques disclosed in U.S. Pat. No. 5,822,029 and other prior art references.
Adding the reflective-type black matrix pattern to the CLC spectral filter structure increases the complexity of the display system and adds to the overall cost of the display which must be minimized for low-cost consumer product applications.
While recommending the use of light collimation techniques to ensure that the incident upon the spectral filter falls within the angular acceptance bandwidth of its CLC material, U.S. Pat. No. 5,822,029 fails to disclose, teach or suggest practical ways of achieving this requirement of CLC material, nor even recognizes in the slightest way the fact that non-collimated light falling on the broad-band CLC reflective polarizer (32) results in significant polarization distortion, as illustrated in FIGS. ID through 1H.
While U.S. Pat. No. 5,822,029 discloses the use of CLC-based spectral filters for improved light recycling within a LCD panel, the methods taught therein necessarily result in CLC films having narrow bandwidths which limited their usefulness in creating practical color (i.e. spectral) filter structures for use in LCD panels. While U.S. Pat. No. 5,822,029 discloses a technique of increasing the bandwidth of the cholesteric liquid crystal material by providing a plurality of different pitches in each portion of the material (e.g. use a thermochromic material and vary its temperature while applying ultraviolet light to fix the material), this method is difficult to use in practice and does not produce good results because the bandwidth of the reflective materials is limited to about 80 nm. Applicants have discovered that for good results, a bandwidth of at least 100 nm is required for CLC-based spectral filters.
The CLC-based spectral filters disclosed in U.S. Pat. No. 5,822,029 do not had a sufficient broad enough spectral bandwidth to reflect all the light needed to made a good quality color reflective filter. Also, since the reflective bandwidth is not large enough in U.S. Pat. No. 5,822,029, only one color at a time can be reflected, thereby requiring that prior art CLC-based spectral filters have at least three color reflecting sections per CLC layer, for a two layers CLC spectral filter structure.
In summary, while it is well known to use CLC-based spectral filters and CLC reflective polarizers within color LCD panel assemblies to improve the brightness of images displayed therefrom, prior art CLC-based LCD panels suffer from several shortcomings and drawbacks relating to: (1) color changes due to viewing angle; (2) controlling the bandwidth of the spectral components to be reflected within the panel for recycling; (3) difficulty in tuning the color-band of spectral components to be transmitted to the viewer for display; (4) difficulty in achieving high contrast between the spectral components in different color bands; (5) difficulty in making CLC-based spectral filter layers which result i n spectral filters having high color purity and a broad color gamut; and (6) realizing a reflective-type black matrix which is inexpensive and does not increase the complexity of the system.
Thus, there is a great need in the art for an improved color LCD panel which is capable of producing high brightness color images without the shortcomings and drawbacks of the prior art LCD panel devices.
Accordingly, a primary object of the present invention is to provide an improved color LCD panel capable of producing high brightness color images, while avoiding the shortcomings and drawbacks of prior art techniques. Another object of the present invention is to provide such a color LCD panel, in which the spatial-intensity modulation and spectral (i.e. color) filtering functions associated with each and every subpixel structure of the LCD panel are carried out using systemic light recycling principles which virtually eliminate any and all absorption or dissipation of the spectral energy produced from the backlighting structure during color image production.
Another object of the present invention is to provide such a color LCD panel, in which image contrast enhancement is achieved through the strategic placement of broad-band absorptive-type polarization panels within the LCD panel.
Another object of the present invention is to provide such a color LCD panel, in which glare due to ambient light is reduced through the strategic placement of a broad-band absorptive-type polarization panel within the LCD panel.
Another object of the present invention is to provide such a color LCD panel, in which a single polarization state of light is transmitted from the backlighting structure to the section of the LCD panel along the projection axis thereof, to those structure or subpanels where both spatial intensity and spectral filtering of the transmitted polarized light simultaneously occurs on a subpixel basis in a functionally integrated manner. At each subpixel location, spectral bands of light which are not transmitted to the display surface during spectral filtering, are reflected without absorption back along the projection axis into the backlighting structure where the polarized light is recycled with light energy being generated therewith. The recycled spectral components arc then retransmitted from the backlighting structure into section of the LCD panel where spatial intensity modulation and spectral filtering of the retransmitted polarized light simultaneously reoccurs on a subpixel basis in a functionally integrated manner.
Another object of the present invention is to provide such a color is LCD panel, in which the spatial-intensity modulation and spectral filtering functions associated with each and every subpixel structure of the LCD panel are carried out using the polarization/wavelength dependent transmission and reflection properties of CLC-based filters.
Another object of the present invention is to provide such a color LCD panel having a multi-layer construction with multiple optical interfaces, at which non-absorbing broad-band and pass-band (i.e. tuned) polarizing reflective panels are used to carryout systemic light recycling within the LCD panel such that light produced from the backlighting structure is transmitted through the LCD panel with a light transmission efficiency of at least 90%.
Another object of the present invention is to provide a novel LCD panel, in which both non-absorbing broad-band and pass-band (i.e. tuned) polarizer filters are used to avoid absorbing or dissipating any of the spectral energy produced from the backlighting structure during image production in order that high-brightness images can be produced using low-intensity backlighting structures.
Another object of the present invention is to provide such a color LCD panel, in which an array of pass-band CLC polarizing filter elements and an array of electrically-controlled liquid crystal elements are disposed between a pair of broad-band CLC polarizing filter panels used to realize the LCD panel.
Another object of the present invention is to provide such a color LCD panel, in which the spectral components of light produced from the backlighting structure are recycled (i) between the spectral filtering array and the backlighting structure, (ii) within the backlighting structure itself, and (iii) among adjacent subpixels within the LCD panel in order to improve the overall light transmission efficiency of the LCD panel.
Another object of the present invention is to provide such a color LCD panel, in which the array of liquid crystal elements can be realized using an array of electrically-controlled birefringent (ECB) elements which rotate the linear polarization state of the transmitted light, or invert the polarization state of circularly polarized light being transmitted through the LCD panel.
Another object of the present invention is to provide such a color LCD panel, in which the backlighting structure thereof can be realized using a light guiding panel based on the principle of total internal reflection, a holographic diffuser based on the principle of refractive index matching and first order diffraction, or other suitable edge-lit backlighting structure which follows in general accordance with the physical principles of the present invention.
Another object of the present invention is to increase the brightness of a LCD panel.
Another object of the present invention is to match the color of the cholesteric color filters in a display to the light input color distribution for effective color separation.
Another object of the present invention is to provide a reflective cholesteric liquid crystal (CLC) color filter with only two different color portions in each CLC layer.
Another object of the present invention is to provide a reflective color filter made from cholesteric liquid crystals that transmits red, green and blue light from different pixels with a two layer configuration, where each layer has only two reflection bandwidths.
Another object of the present invention is to improve the contrast between the colors.
Another object of the present invention is to provide a black matrix by overlapping two layers of reflective color filters.
Another object of the present invention is to provide a color filter which contains red, green, blue and transparent sub-pixels.
Another object of the present invention is to use cholesteric liquid crystal reflective color filters to transmit any polarization light of desired colors.
Another object of the present invention is to eliminate the quarter wave plate in a reflective color filter display.
Another object of the present invention is to more easily make reflective Cholesteric Liquid Crystal color filters.
Another object of the present invention is to increase the patterned color section size thus making the pixels easier to make by using only two colors per layer without losing the display resolution.
Another object of the present invention is to reduce the boundary effects of pixels by having larger size patterned color sections per layer.
Another object of the present invention is to tune the color filter to the desired center wavelength and bandwidth for better color control.
Another object of the present invention is to eliminate a layer in a display solely for creating a black matrix.
Another object of the present invention is to reduce the cost of making reflective color filters.
Another object of the present invention is to improve the performance of reflective color filters.
Another object of the present invention is to produce pitch gradient broadband reflective cholesteric liquid crystal materials with different central bandwidths.
Another object of the present invention is to reliably and cost effectively produce broadband reflective cholesteric liquid crystal materials with different central bandwidths.
Another object of the present invention is to make layers of reflective cholesteric liquid crystal materials with two separate portions having two separate bandwidths and central wavelengths.
Another object of the present invention is to have one layer of cholesteric liquid crystal materials with each side of the layer reflecting a different band of wavelengths around a different central wavelength.
Another object of the present invention is to expose one half of a layer to UV light which is absorbed by attenuation in the cholesteric liquid crystal materials by the time it is one half way through the layer, thus polymerizing one half of the layer.
Another object of the present invention is to reduce the number of steps in the process of making a reflective cholesteric liquid crystal color filter by reflecting two different bandwidths of light one bandwidth in the top portion of the layer and one bandwidth in the bottom portion of the layer.
Another object of the present invention is to eliminate the steps of aligning and gluing two layers together in making a reflective cholesteric liquid crystal color filter.
Another object of the present invention is to have stack several different polymerized states of cholesteric liquid crystal materials in one layer material.
Another object of the present invention is to provide a quarter wave plate integral with the reflective cholesteric liquid crystal color filter layer.
Another object of the present invention is to provide a reflective polarizer integral with the reflective cholesteric liquid crystal color filter layer.
Another object of the present invention is to stack multiple portions of polymerized cholesteric liquid crystal materials with different properties in each portion in one layer of cholesteric liquid crystal material.
Another object of the present invention is to develop material recipes for creating the reflective color filter made from the cholesteric liquid crystal that transmits red, green and blue from different sub-pixels with a two-layer configuration each of which has only two reflection bandwidths.
Another object of the present invention is to provide fabrication processing procedures for creating the reflective color filter made from cholesteric liquid crystals that transmits red, green and blue from different subpixels with a two-layer configuration each of which has only two reflection bandwidths.
Another object of the present invention is to polarize unpolarized incident light in reflection mode for a large bandwidth and a wide range of incident angles.
Another object of the present invention is to polarize unpolarized incident light in transmission mode for a wide range of angles and a large bandwidth.
Another object of the present invention is to analyze circularly polarized light for a wide range of angles and a large bandwidth in reflection mode.
Another object of the present invention is to analyze circularly polarized incident light in transmission mode for a wide range of angles and a large bandwidth.
Another object of the present invention is to transmit broadband polarized light without spectral distortions for a large range of angles.
Another object of the present invention is to compensate for the color change associated with using reflective CLC polarizers.
Another object of the present invention is to compensate for elliptical distortions of circularly polarized light in cholesteric liquid crystals when incident light is at large angles.
Another object of the present invention is to compensate for spectral distortions of circularly polarized light in cholesteric liquid crystals when incident light is at large angles.
Another object of the present invention is to compensate for elliptical distortions of circularly polarized light in cholesteric liquid crystals when the light is viewed at large viewing angles.
Another object of the present invention is to compensate for color distortions associated with polarization distortions of circularly polarized light in cholesteric liquid crystals when the light is viewed at large viewing angles.
Another object of the present invention is to compensate the severe degradation in polarization behavior at large incident angles associated with CLC-based broadband polarizers.
As a result of the present invention, improved LCD panels can be made having reflective color filters which offer significant advantages over absorptive color filters in that they reflect light for recycling in the system rather than convert the light to unwanted heat as absorptive color filters do. Such a reflective color filter system can enhance the brightness of the display and reach near 100% utilization efficiency. The efficiency derives from reflecting color filters which reflect light within the system for recycling rather than absorb the light.
By using reflective color filters of the present invention, the brightness of the LCD display is increased, the cooling systems needed to expel waste heat are eliminated, the power consumption is reduced allowing for smaller batteries and longer life per charge while reducing the weight and size of the display and lowing its cost.
In order to make simpler reflective color filters to transmit red, green, and blue for pixels in a display, an novel architecture for having two color reflecting portions per layer has been devised. In one embodiment only one film layer is needed because two colors can be reflected by the same film layer when the top part of the layer reflects one color and the bottom part of the film reflects another color.
In another embodiment, image displays are provided having three primary colors for color images, pixel arrays with three sub-pixels for transmitting blue, red and green are used. With a two layer system blue and green reflective layers transmit red, red and blue reflective layers transmit green, and green and red reflective layers transmit blue. Clear sub-pixels transmit the colored light incident thereon. The clear sub-pixels can reflect ultraviolet and infrared light.
In other embodiments of the present invention, if a portion of a broadband layer reflecting blue and green is paired with a portion of a layer which is clear, then red light is transmitted. When a portion of a broadband layer reflecting green and red is paired with a portion of a layer which is clear blue is transmitted. When a portion of a layer which is blue reflecting is paired with a portion of a layer which is red reflecting, green is transmitted. When the green and red portion in one layer overlaps part of the blue portion and part of the blue and green portion in the other layer, a black matrix is formed in the overlap portion.
The bandwidth of the reflective cholesteric liquid crystals is controllable for wide bandwidths by using a pitch gradient tuning. Using this method layers of cholesteric liquid crystals reflecting adjacent colors such as both blue and green and both green and red are used to precisely tune the colors reflected and transmitted by the display. Using the wider bandwidths allows layers with two color reflecting portions per layer to be constructed instead of three color portions per layer as in the prior art.
When left handed and right handed CLC color filters having the same structure are both used, white incident light of any type polarization on the pixels is converted to red, green, and blue without polarization state distortion. This color filter works for all polarizations from linear, to circular and even to unpolarized light. When linearly polarized light is incident on such color filters which are used in conjunction with conventional twist nematic or super twist nematic in liquid crystal displays the need for a quarter wave plate that converts circularly polarized light to linear light is eliminated. This simplifies the display system, makes; fabrication easier, reduces the cost, removes the color chromaticity problem associated with the limited bandwidth of the wave plate and increases the displays contrast ratio by eliminating light losses due to quarter wave plate bandwidth limitations.
If just left handed or just right handed circularly polarized light is transmitted from the color filters, a quarter wave plate can be used to make the light linearly polarized for use in displays in conjunction with twisted nematic cells to turn the transmitted light on and off from the viewer in conjunction with a linear analyzer.
A black matrix of one form or another is necessary in all the liquid crystal displays (LCD). The matrix used in the reflective system is formed by using overlapping reflective color filters to reflect all incident light. Therefore no added layers of reflective materials or light absorbing blocking masks are necessary to create the black matrix.
The portion of the layer having the same reflective color is enlarged by arrangement of the pixels with like color portions adjacent to have larger easier to make color portions on each layer. This reduces the cost of manufacture of the LCD.
A polarization preserving diffuser is used with a light collimator to increase the viewing angle of the LCD while keeping the color distortion of the color filter at a minimum level.
These and other objects of the present invention will become apparent hereinafter and in the Claims to Invention.